Growth Hormone Releasing Hormone Receptor-Immunoreactive Cells Increase in the Barrel Field in Response to Whisker Deflection in Rats.

The homeostatic regulation of sleep involves several sleep regulatory substances including growth hormone releasing hormone (GHRH). Previous studies showed that microinjection of GHRH onto the surface of the cortex increases EEG delta power during non-REM sleep and that cortical GHRH receptor mRNA levels increase after 6 h of sleep deprivation during the dark. These studies suggest that the GHRH receptor may be sensitive to stimulation of whiskers that are mainly used during the dark. Whisker deflection increases fos expression within specific barrels of the primary somatosensory cortex providing an anatomical localization for a specific neuronal activation without invasion into the brain. We hypothesized that activation of neuronal circuits enhances release of sleep regulatory substances locally and such use-dependent molecules provide a mechanism for state-specific EEG slow wave power and for micro circuitry connectivity processes such as synaptic scaling. Six male Sprague-Dawley rats (200-300g) were stimulated unilaterally by brushing with fingers the long whiskers along the caudal edge of the whisker field for 2 h in the afternoon. After 2 h of stimulation, the rats were perfused with 4% paraformaldehyde, the brains post-fixed for 2 h, sunk in 20% sucrose and immunoreactivity (IR) for GHRH receptor (GHRHR), fos and nerve growth factor (NGF) were analyzed. In layer IV (the primary sensory input to the barrel field) as well as in layers II-III, the number of GHRHR-IR cells increased in the barrel columns that showed fos activation in an adjacent section. No changes in IR for NGF were observed in adjacent sections. These data suggest that afferent activation of a cortical column enhances the number of GHRH receptors in these cortical cells. These data support our hypothesis that the GHRH receptor is functionally sensitive to neuronal activation and are consistent with the Krueger-Obal theory of use-dependent local sleep.